Added some additional helpers for both Range and Position

[david-driscoll]

Ported from vscode. These are to help when working with and on ranges.

[codecov]

Codecov Report

Merging #410 into master will increase coverage by 0.40%.
The diff coverage is 85.61%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   72.53%   72.93%   +0.40%     
==========================================
  Files         569      575       +6     
  Lines       13780    14202     +422     
  Branches     1292     1366      +74     
==========================================
+ Hits         9995    10358     +363     
- Misses       3785     3844      +59     

Impacted Files	Coverage Δ
src/Protocol/Models/Position.cs	63.15% <ø> (ø)
src/Testing/TestConfigurationProvider.cs	68.75% <0.00%> (ø)
src/Testing/PositionMarker.cs	36.84% <36.84%> (ø)
src/Testing/TestSourceHelpers.cs	81.39% <81.39%> (ø)
src/Protocol/Models/Range.Helpers.cs	89.10% <89.10%> (ø)
src/Testing/TestContent.cs	95.31% <95.31%> (ø)
src/Protocol/Models/Position.Helpers.cs	100.00% <100.00%> (ø)
src/Protocol/Models/Range.cs	73.33% <100.00%> (+5.33%)	⬆️
src/Testing/TestContentOptions.cs	100.00% <100.00%> (ø)
src/Protocol/Models/RenameRegistrationOptions.cs	62.50% <0.00%> (-37.50%)	⬇️
... and 8 more

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